Band edge emission enhanced organic light emitting diode with a localized emitter

ABSTRACT

A light emitting photonic crystal having an organic light emitting diode and methods of making the same are disclosed. An organic light emitting diode disposed within a photonic structure having a band-gap, or stop-band, allows the photonic structure to emit light at wavelengths occurring at the edges of the band-gap. Photonic crystal structures that provide this function may include materials having a refractive index that varies.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. Ser. No. 16/885,401 filed May28, 2020, which is a continuation of U.S. Ser. No. 15/738,214 filed onDec. 20, 2017 issued as U.S. Pat. No. 11,139,456 on Oct. 5, 2021, whichis a 371 National Stage of International Application No.PCT/US2016/38479, filed Jun. 21, 2016, which was published asInternational Publication No. WO 2016/209797, and which claims thebenefit under 35 U.S.C. § 119(e) of the earlier filing date of U.S.Provisional Patent Application No. 62/183,771 filed on Jun. 24, 2015,the disclosures of which are incorporated by reference herein.

BACKGROUND

The following description relates to an improved light emitting deviceand methods of manufacturing the same.

Organic light-emitting diodes (“OLEDs”) are optoelectronic devices madeby placing a layer of organic material between two electrodes, whichwhen a voltage potential is applied to the electrodes and current isinjected through the organic material, visible light is emitted from theorganic material or emissive material. Due to the high power efficiency,low cost of manufacture, lightweight and durability, OLEDs are oftenused to create visual displays for portable and non-portable devices aswell as consumer lighting.

OLEDs are rapidly replacing liquid crystal display (“LCD”) devices inthe market for lighting and display devices. This is driven byadvantages in viewing experience, size, weight, and simplicity. In U.S.Pat. Nos. 7,335,921, 9,129,552 and U.S. Patent Application PublicationNo. 2004/0069995, light emitting diode (“LED”) devices and particularlyOLEDs are described in which one or more feedback means are integratedwith the light emitting diode structures so as to provide enhancedlevels of light emission and energy efficiency by exploiting phenomenonof stimulated emission. These devices are collectively referred to asfeedback enhanced organic light emitting diodes (FE-OLEDs). U.S. Pat.No. 9,129,552 application describes methods of utilizing FE-OLEDs indisplay devices. Generally, FE-OLED architectures are useful fornonlaser and laser applications. Since light is emitted by a FE-OLEDthrough emission stimulated by feedback light, such emitted light ispredominantly collimated and emitted normal or near normal to thesurface of the device.

A great advantage of non-lasing FE-OLEDs is that the energy efficiencyof the devices is greatly enhanced vis-a-vis conventional OLEDs. Themajority of light produced in conventional OLEDs never emerges fromthese devices because it is emitted at angles far enough from the normalto the device surface that it is trapped in the device by internalreflections. Since essentially all the light emitted by FE-OLEDs isemitted at near normal angles, nearly all the light that is producedwithin the devices escapes. This can result in a three-fold increase inenergy efficiency.

A distinguishing feature of FE-OLEDs, is a resonant cavity formedbetween two feedback means, for example two photonic crystals. Withinthis resonant cavity a layer of emitter material is disposed which emitsphotons as a result of stimulated emission induced by photons that arelocalized within the resonant cavity by the opposing feedback means. Incertain of these embodiments a complete OLED is formed within theresonant cavity. By tailoring the feedback means to redirect the lightemitted by the emitter material, or emitter, back towards and throughthe emitter material, the emitted light is localized within the resonantcavity—that is, it is cycled back and forth between the two feedbackmeans, repeatedly passing through the emitter material increasing theprobability of inducing further stimulated emission in the emittermaterial. By tailoring the location of the emitter between the feedbackmeans in order to localize the peak optical power of the light at theemitter material in this way, the efficiency of light emission isenhanced and resulting devices are an improvement over known OLEDs. As aresult, the amount of optical power output from the emitter materialwithin the resonant cavity is highly efficient in terms of theelectrical power applied to the device. Furthermore, because thestimulated emission occurs normal to the resonant cavity interface, andtherefore normal to the FE-OLED emission surface, very little light islost by emission into the plane of the emission material or to totalinternal refraction at the interface boundaries. Methods for thefabricating feedback means are described in U.S. Patent ApplicationPublication No. 2004/0069995 (the “'995 Application”), which isincorporated herein by reference in its entirety.

A drawback in known FE-OLED devices is their difficulty to manufactureon any scale due to demanding physical constraints on device design.While FE-OLEDs provide significant improvements over prior art LEDs andOLEDs, a light emitting device is desired which will increase theoptical efficiency and provide the versatility and benefits of OLEDswhile simplifying the device design process and allowing high yieldmanufacturing processes, while reducing heat produced, consuming lesspower overall, enabling tuning across a broad range of emissionfrequencies, and lasting longer than presently available OLED and LEDlight sources.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the basic structure of an organic light emittingdiode.

FIG. 2 illustrates index of refraction profile of a simple model defectmode device.

FIG. 3 illustrates the optical intensity of the light distributedthrough a defect mode device overlaying the defect mode device resonantcavity.

FIG. 4 illustrates the density of states spectrum in a medium having aphotonic refractive index profile and corresponding transmissivityspectrum.

FIG. 5 illustrates the index of refraction profile of a model band-edgeemitting photonic structure device.

FIG. 6 illustrates the optical intensity of the light distributedthrough a band-edge emitting photonic structure device.

FIG. 7 illustrates various embodiments of the present invention alongwith an associated refractive index profile of the various embodimentsdescribed.

FIG. 8 illustrates various embodiments of the present invention.

FIG. 9 illustrates various embodiments of the present invention.

FIG. 10 illustrates various embodiments of the present invention.

FIG. 11 illustrates design considerations of a band-edge emittingphotonic structure device.

DETAILED DESCRIPTION OF THE INVENTION

A photonic crystal is a material having a structure with periodicvariation in the value of its refractive index which gives rise to aband-gap, or stop-band. This is referred to as a photonic crystalexhibiting a photonic band-gap, or a photonic crystal having a band-gap.A photonic band-gap or stop-band is a range of wavelengths in thetransmission spectrum of a medium in which electromagnetic radiation(EMR) cannot propagate through the structure in the direction ofrefractive index variation, that is to say, light cannot propagate inthat direction. Photonic crystals can be designed such that therefractive index variation and thus the stop-band is in a singledirection or dimension, i.e. a distributed Bragg reflector (“DBR”), orin two or three dimensions, as disclosed in among other places the '941Application. In describing a stop-band in terms of wave propagationmodes, a photonic crystal is a structure in which electromagnetic wavemodes over a band of wavelengths are not allowed. The range ofwavelengths prohibited from propagating through the photonic crystal isdictated by the periodicity of the refractive index variation in thephotonic crystal structure. This band-gap, or stop-band, is theelectromagnetic analog to a crystalline atomic lattice acting on anelectron wavefunction within a semiconductor to produce the band-gaps ofsolid-state physics. That is, within the photonic crystal particularsolutions to the EMR wave equation are prohibited within a certain rangeof wavelengths. This range of wavelengths, within which no solutions tothe wave equation exist, is the band-gap or stop-band. The stop-band fora particular structure is tailored to prohibit light having a range ofwavelengths from propagating along a transmission axis by adjusting theperiodicity of the photonic crystal itself in the direction of thetransmission axis. In the case of a DBR this can be accomplished byadjusting the optical thickness of its constituent layers.

Photonic crystals can be created in a number of ways, for example the'995 Application and U.S. Pat. No. 7,335,921 (the “'921 Patent”)disclose a number of alternative forms photonic crystals can take. The'921 is herein incorporated by reference in its entirety. Photoniccrystals with stop-bands can be produced utilizing DBRs and, inaddition, can be produced utilizing holographically recorded gratings,opals, inverse opals, aligned nematic liquid crystals, chiral liquidcrystals, and potentially polymerized lyotropic liquid crystalstructures.

Conventional OLEDs, an example 100 of which is portrayed in FIG. 1 , aregenerally comprised of a number of functional layers each of which serveto improve the functionality of the device. For example, the OLED 100consists of a substrate 110, a transparent anode 120, a hole injectionlayer 130, a hole transporting layer 140, an emitter layer 150, anelectron transporting layer 160, and a metal cathode 170. The device 100functions as follows: when an electrical potential difference is appliedbetween the anode and the cathode, positively charged holes are injectedfrom the anode 120 into the hole injection layer 130. Under theinfluence of the imposed electric field the holes flow from the holeinjection layer, through the hole transporting layer 140 and into theemitter layer 150. At the same time electrons are injected from thecathode 170 into the electron transporting layer 160. Under theinfluence of the imposed electric field the electrons flow from theelectron transporting layer into the emitter layer 150. In the emitterlayer the electrons and holes pair together on single organic moleculespromoting the molecules into electronically excited states. Theseexcited states (excitons) then collapse to emit photons, or in otherwords to produce light. It will be appreciated that methods andapparatus for applying a potential between the anode and cathode of anOLED for a particular application are known.

In OLEDs the hole injection layers 130, the hole transporting layers140, the emitter layers 150, and the electron transporting layers 160are all composed of organic materials. A typical device may also containadditional layers such as electron injection layers, hole blockinglayers and electron blocking layers. Generally, the organic materialsfor forming the various layers of an OLED have quite low charge carrier(electron or hole) mobilities. As a result, if these layers are at allthick, very large impedance losses will occur in the OLEDs causing thedevices to run at very elevated voltages and suffer thermal failureleading to a short device lifetime.

FE-OLED, also known as defect-mode devices, exploit the phenomenon ofstimulated emission by creating a high photon density within a cavity,or defect, between two feedback means, where the cavity contains anemissive material, for example the emitter of an OLED. The feedbackmeans may be two photonic crystals, wherein the stop-bands are tailoredto reflect light emitted by the emissive material back towards theemissive material. Alternatively, the feedback means may be a singlephotonic crystal opposite a metallic reflector or mirror, or, thefeedback means be two holographically recorded materials gratings. Asdescribed above, FE-OLEDs provide many improvements over traditionallight emitting devices used in displays; however FE-OLEDs suffer forbeing difficult to mass produce.

To illustrate the difficulties, consider, a simplified model (not shown)of such an FE-OLED which consists of two DBR's separated by a cavity,such that the DBR surfaces are parallel to each other. Also, considerFIG. 2 illustrating a refractive index profile 200 experienced by lighttravelling through such a model FE-OLED device parallel to transmissionaxis 220; and, consider FIG. 3 illustrating the resulting lightintensity distribution 270 within the device. Within the cavity, havinga refractive index profile 250, of such a device, an OLED (not shown),or portions thereof may be formed. Ideally the OLED is formed such thatthe emitter material of the OLED is precisely aligned along a plane 210within the cavity, plane 210 ideally experiencing maximum lightintensity in order to maximize further stimulated emission within theemitter material. In such a device, light travelling through the devicewill experience a changing index of refraction along a transmission axis220. This changing index of refraction will be periodic passing throughthe first portion of the photonic crystal, corresponding to refractiveindex profile 240, or through the second portion of the photoniccrystal, corresponding to refractive index profile 260, but thisperiodicity will be interrupted by the cavity, which will cause a phaseslip. The model device consists of a stack of 43 layers (not shown),giving rise to the refractive index profile 230. The first section ofthe profile 240 corresponds to a stack of eleven 43.98 nm thick layersof a transparent material having a refractive index of 2.70, andinterposed between these layers are ten layers of a transparent materialeach 59.38 nm thick having a refractive index of 2.00. The layersalternate from high to low refractive index through the stack and eachof the layers has a physical thickness such that its optical thickness(refractive index multiplied by physical thickness) is equal to 118.8nm, or a quarter wave thickness for light having wavelength of 475 nm.On top of the last of these 21 layers is a 148.44 nm thick layer 250 ofa material having a refractive index of 1.60. On top of this layer is asecond feedback means 260 consisting of 21 more layers identical to thefirst 21 in the stack. In this model light emission is assumed to occurin a plane 210 at the center of the central 148.44 nm thick layer. Insummary, FE-OLEDs are embodied as two feedback means, e.g. two photoniccrystals (e.g. 240 and 260) with the emitter layer of an OLED located ina cavity 250 between the two feedback means.

FIG. 3 shows a plot 270 of the modeled light intensity distributionthrough the FE-OLED, described above, along transmission axis 220 ofFIG. 2 , when the OLED emits light having a wavelength of 475 nm intothe stack. It can be seen that there is a strongly peaked maximum oranti-node of light intensity at the center of the central 148.44 nmthick layer, corresponding to plane 210, with nodes of zero intensity atthe boundaries 280 a and 280 b of this central layer. Thus the twotwenty-one layer feedback structures are reflecting light back into thecavity formed by the 148.44 nm thick central layer.

The operating principle of FE-OLEDs, is that the high photon densitywithin the cavity results in very efficient stimulation of lightemission from the emitter in the cavity as long as the emitter iscentered precisely on plane 210. Difficulties with devices of this typearise because the maximum light intensity occurs in a very narrow regionwithin the cavity, and light intensity rapidly drops off moving awayfrom plane 210. In the modeled example shown in FIG. 2 and FIG. 3 thisplane is centered in the cavity between the feedback layers. However, inthis simplified model the material in the cavity has a uniformrefractive index.

In practice, generally this is not the case, because multiple OLEDfunctional layers, comprising different materials with differentrefractive indices, lie within the cavity 250 and this results in adifferent, more complicated, distribution of light intensity. Thethickness and location of the layers that make up the OLED are largelydictated by electronic considerations and thus it may not be possible tolocate an emitter layer at the plane 210 where maximum light intensityoccurs. If it is possible, registering the emitter layer to occur in thelocation of maximum light intensity 210 is a difficult task. A furtherissue is that the strength of the light intensity localization in thecavity of these devices is very sensitive to the cavity thickness andcould vary considerably from device to device if the OLED's organiclayer thicknesses are not held to very tight tolerances. Referring backto FIG. 2 and the device upon which the profile 230 is modelled, becausea cavity is necessary for light localization, the resonant cavity 250and thus the emitter layer, and the functional and non-functional layerssurrounding the emitter layer in various embodiments, do not (andcannot) function as part of the photonic crystal itself. Thus,defect-mode devices are embodied as two feedback means, e.g. with indexof refraction profiles described by 240 and 260, e.g. two photoniccrystals, with the emitter layer of an OLED located in a cavity 250between the two feedback means.

In summary, up until the present time the commercial potential ofFE-OLEDs has not been able to be realized because of yield issues havingto do with maintaining proper layer thicknesses, the difficulty inspatially registering the two photonic crystal structures required oneto the other, and the difficulty in registering the emitter layer at thepeak optical power of the light distribution in the device; which peakoptical power is itself dependent on the interplay between the period ofthe refractive index in each feedback means, the thickness of thecavity, the special registration of the two feedback means and thevariations in index of refraction that arise in practice within thecavity caused by the presence of the OLED layers. Given the difficultyin manufacturing FE-OLED type devices, it is similarly difficult tomodify the manufacturing processes to provide multiple colors of light(i.e. the desired output spectrum), because modifying the periodicity ofthe index of refraction variation in order to shift the stopband alsorequires modification of the size of the resonant optical cavity betweenthe two devices, reregistering the phase of the two feedback means oneither side of the resonant cavity and also reregistering the locationof the emitter layer within the resonant optical cavity such that itfalls on the peak optical power 210.

A second type of device enhancement based on stimulated emissions, whichexploits phenomena manifested at the EMR modes found at the spectraledges of a photonic stop-band, is disclosed. This second type of deviceis referred to as a band-edge emission device. A band edge emissionarises when light is emitted inside a photonic crystal structure. Whenan emitter material is disposed within a photonic crystal and excited,for example by application of a voltage (electric pumping) or byoptically pumping, to emit light having a band of wavelengths some ofwhich are overlapped by the stop-band, light is prohibited frompropagating through the photonic structure in that band. Instead lighthaving wavelengths at the edge of the stop-band or band-gap will beemitted into the photonic crystal and then emerge from a surface orsurfaces of the photonic crystal structure. In various embodiments ofthis type of device an emitter material is disposed entirely within onelayer of a DBR constituting a photonic crystal.

It is tempting to assume that if an emitter material is introducedwithin a photonic crystal, wherein some portion of the emission spectrumof the emitter material is overlapped by the photonic stop-band of thephotonic crystal, that the emission modes or states that would exist infree space, but for the photonic crystal, are destroyed. However, suchemission modes are only expelled from, or prohibited from existingwithin, the photonic crystal and instead of being destroyed can beenvisioned as being ‘stacked-up’ at the edges of the stop-band. In termsof density of states, the number of allowable wave propagation states ormodes per interval of frequency in the EMR spectrum increasessubstantially at the edges of the stop-band.

As described above, spontaneous emission is suppressed for wavelengthsof light within a photonic band-gap. This is because the probabilitythat an excited state atom is de-excited through either spontaneousemission or stimulated emission is proportional to the density of photonstates, which vanishes throughout the photonic band-gap for given modes.For an illustrative example, FIG. 4 illustrates the density of statesspectrum 400 of an emission medium disposed within a photonic crystal,and the transmission spectrum 402 for a photonic crystal having astop-band or band-gap 404 in which the density of states goes to zeroand so prohibits the propagation of modes having frequencies fallingwithin the band-gap 404, i.e. between frequencies B and C. Overlayingthis spectrum is the free space density of states spectrum 414 of theemissive material existing outside the photonic crystal. In terms of EMRfrequency, frequency B forms the lower boundary 410 of stop-band 404 andfrequency C forms the upper bound 412 of stop-band 404. Because theamount of light that an emitting molecule will emit into itssurroundings depends on the density of states available to propagate thelight, an emitter molecule emitting into a surrounding photonic crystalhaving a density of states spectrum 400 will emit considerably morelight photons at frequencies falling in the lower range 406, betweenfrequencies A and B, or in the upper range 408, between frequencies Cand D. Additionally, photonic crystals have the benefit of being atleast partially transparent to band-edge frequencies as is seen by thetransmissivity spectrum 402 showing decreasing transmissivity throughthe lower region 406 and then increasing transmissivity through theupper region 408 as the frequency increases, thus allowing band-edgeemission to escape the device.

Referring to FIG. 4 , when EMR modes having frequencies falling withinthe stop-band 404 are induced by an emitter material within a photoniccrystal having stop-band 404 the density of states for EMR withinstop-band 404 are suppressed and tend towards zero, while the density ofstates for modes of light having frequencies approaching the lower bound410 from the left, or approaching the upper bound 412 from the right,increases substantially, in particular for modes of light havingfrequencies in the lower range 406, between frequencies A and B, and inthe upper range 408, between frequencies C and D. These emissions inlower range 406 and upper range 408 are known as band-edge emissions. Ascan be seen from FIG. 4 there is an increased density of states, inrelation to free space (e.g. 414), extending beyond ranges A-B and C-D,thus the ranges A-B and C-D could be altered outwardly from thestop-band and still be described as encompassing band-edge emissions.

A band-edge emission having a frequency spectrum centered on a frequencyν_(AB) in lower range 406 corresponds to EMR having a wavelengthspectrum centered on wavelength λ_(AB)=(c/ν_(AB)) which corresponds tomodes of light having wavenumber k_(AB)=(2π/λ_(AB)); and similarly aband-edge emission having frequency spectrum centered on ν_(CD) in upperrange 408 corresponds to EMR having a wavelength spectrum centered onλ_(CD)=(c/ν_(CD)) which corresponds to modes of light having wavenumbersk_(CD)=(2π/λ_(CD)). A band-edge emission spectrum centered on ν_(AB)then is comprised of photons having a statistical distribution of energycentered on E_(AB)=hν_(AB)=h (c/λ_(AB)), (h being Planck's constant).Light in these devices with a frequency ν_(AB) equivalent to wavelengthλ_(AB) will have a very large number of modes in the small band ofwavenumbers centered on k_(AB). Modes having the wavenumber k_(AB) canhave varying phases φ. Whereas, in FE-OLED devices the modes all havenodes locked onto the cavity edges, e.g. as in FIG. 3 , and thus thereis a single mode for each allowed wavenumber, in band edge devices, manymodes with the same wavenumber are allowed and this means that the plotof photon density versus position through the device is smooth with nonodes, as discussed below. As a result variations in emitter layerposition within a photonic crystal have little effect in a band edgedevice.

Because of the partial transmissivity of a photonic crystal to theseband-edge modes, light emitted at these wavelengths builds up within themedium comprising the band-edge photonic crystal. The combination of thehigher than normal level of photon emission at wavelengths adjacent tothe stop-band combined with buildup of these photons within the mediumdue to internal reflections yields very high photon densities throughoutthe bulk of the photonic crystal medium. These high photon densitiesensure further stimulated emission from essentially all of the excitedstate emitter molecules embedded within the photonic crystal. In aone-dimensional photonic crystal, the direction of propagation ofstimulated emission photons is normal to the emission surface.

Until now solid state band-edge organic light emitting diodes (BE-OLEDs)have not seemed to be possible because of the spatial mismatch betweenthe assumed distribution of photons in the photonic crystal structureand the much smaller volume occupied by the emitter material. It wasaccepted that without doping emitter material throughout the extent ofthe photonic crystal structure, as had been the case in previouslyproduced band-edge lasers, the insufficient interaction of previouslyemitted light entrained in the photonic crystal with excited emittermolecules would preclude a useful and efficient light emitting device.

Surprisingly a new light emitting device containing an OLED embedded ina single thin layer within a single photonic crystal will function as anon-lasing band-edge emitting light emitting device while providing asubstantial amount of emitted light. Moreover, such a device providessubstantially increased efficiency over conventional LEDs, OLEDs andcavity type FE-OLEDs. Optical power output on the order of 300 lumensper watt has been achieved. This increased efficiency is realized whilehaving the benefit of being simpler to produce and generating less heat.Disclosed devices additionally provide a very refined emission spectrumoccurring predominantly within a relatively narrow range of frequenciesoccurring close to the band edge of the photonic stop-band of thephotonic crystal. The manufacture of these devices also allows adesigner to easily fine tune the output frequency (i.e. color of theemitted light) of the resulting device by varying only a singlefactor—the periodicity of the index of refraction.

This allows the fabrication of devices that emit highly saturated colorsof selected wavelengths because of their relatively narrow emissionspectrum. As a result, full-color, OLED displays fabricated with red,green, and blue pixels utilizing band-edge emission can replicate a widegamut of colors across the CIE color space.

Enhancements and features of the present invention and methods ofaccomplishing the same may be understood more readily by reference tothe following detailed description of example embodiments and theaccompanying drawings. The present invention may, however, be embodiedin many different forms and should not be construed as being limited tothe embodiments set forth herein. Rather, these embodiments are providedso that this disclosure will be thorough and complete and will fullyconvey the concept of the invention to those skilled in the art, and thepresent invention will only be defined by the appended claims, andequivalents thereof. Thus, in some embodiments, known structures anddevices are not shown in order not to obscure the description of theinvention with unnecessary detail. Like numbers refer to like elementsthroughout. In the drawings, the thickness of layers and regions areexaggerated for clarity.

It will be understood that when an element or layer is referred to asbeing “on,” or “connected to” another element or layer, it can bedirectly on or connected to the other element or layer or interveningelements or layers may be present. In contrast, when an element isreferred to as being “directly on” or “directly connected to” anotherelement or layer, there are no intervening elements or layers present.As used herein, the term “and/or” includes any and all combinations ofone or more of the associated listed items. Expressions such as “atleast one of,” when preceding a list of elements, modify the entire listof elements and do not modify the individual elements of the list.Further, the use of “may” when describing embodiments of the presentinvention refers to “one or more embodiments of the present invention.”When discussing thicknesses or lengths of physical components orportions of the inventive device embodiments in terms of a wavelength oflight, the thickness is such that light passing through such a componentexperiences an equivalent optical thickness (i.e. refractive index timesphysical thickness). For example a physical length equal to an opticalthickness of ¼ of the wavelength of emitted light in a medium having anindex of refraction of 1.1 where the emitted light is 600 nm, wouldresult in an optical thickness of 150 nm, or a physical thickness equalto the optical thickness divided by 1.1, thus a physical thickness of136.36 nm. One of skill in the art will appreciate when such anequivalent thickness is necessitated by the function of the componentbeing described and where thickness and optical thickness may be usedinterchangeably.

Spatially relative terms, such as “below,” “beneath,” “lower,” “above,”“upper,” and the like, may be used herein for ease of description todescribe one element or feature's relationship to another element(s) orfeature(s) as illustrated in the drawings. It will be understood thatthe spatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the drawings.

Embodiments described herein will be described referring to plan viewsand/or cross sectional views by way of ideal schematic views of theinvention. Accordingly, the example views may be modified depending onmanufacturing technologies and/or tolerances. Therefore, the embodimentsof the invention are not limited to those shown in the views, butinclude modifications in configuration formed on the basis ofmanufacturing processes. Therefore, regions shown in the drawings haveschematic properties, and shapes of regions shown in the drawings areexamples of specific shapes of regions of elements and do not limitaspects of the invention

In one embodiment the device disclosed herein may be a unitary photoniccrystal having an emitter material disposed within the photonic crystal.In various embodiments the emitter material may be located in theemitter layer of an OLED. Also, in various embodiments, the unitaryphotonic crystal may be a DBR, and may consist of a series of layers ofvarious materials having different indexes of refraction, arranged insuch a way that light passing through the device experiences asubstantially periodic index of refraction profile. The index ofrefraction profile may be a periodic square wave type function such asthat shown in FIG. 5 , or in other embodiments it may be a continuouslyvarying substantially periodic index of refraction.

The OLED may be formed as a single thin layer. In the case where thephotonic crystal is a DBR, the OLED may alternatively comprise a singlelayer or one or more of the alternating index layers. In a device ofthis type the emitter layer and the associated electric charge carriertransporting and electric charge carrier blocking layers generally foundin OLEDs combine together to form one or more active layers or activezones. Alternatively the OLED components are contained within aninactive material having appropriate index of refraction, wherein theOLED components and the material together form an active layer or activezone. As used herein, active zone and active layer and active region areused synonymously and refer generally to any part of photonic crystalcontaining aspects or components which function as a an OLED,irrespective of its geometry, excepting that the index of refractionexperienced by light passing through the photonic crystal in aparticular direction experiences a substantially continuously varyingperiodic index of refraction that is substantially uninterrupted passingthrough the geometry containing the active region. As used herein,inactive material means any material that does not relate to theelectrical functioning of the OLED device. The active region maycomprise in part inactive material. In one embodiment, the photoniccrystal comprises a DBR consisting of a stack of dielectric layershaving an alternating index of refraction from one layer to the next(e.g. high, low, high, low, high, low, etc.) and the active layer orlayers are λ/4 in optical thickness where λ is a wavelength of thecentral frequency of the stop-band of the photonic crystal, and theindex of refraction of the active layer containing emitter material islower than the index of refraction of each adjacent layer.Alternatively, the index of refraction of the active zone, or layer, ishigher than the index of refraction of each adjacent layer. In anotherembodiment there are two active zones adjacent to each other, one activelayer having a higher index of refraction than the other. In anotherembodiment there are three active layers, one comprising an anode, onecomprising an OLED minus its electrodes, and one comprising a cathode,wherein the anode layer and the cathode layer are each adjacent to theOLED layer. Additionally there may be additional active regions or zonesor layers throughout the photonic crystal. In any case in which thephotonic crystal is a DBR the pattern of alternating high and lowrefractive index layers or zones of λ/4 optical thickness characteristicof photonic crystals carries on through the entire device stack,including the active layer(s), thus forming a single photonic crystalstructure that emits light at the band-edge when the anode and cathodeare energized. More generally, the periodicity of the periodic index ofrefraction is substantially uninterrupted throughout the light emittingphotonic crystal.

In these devices the interaction of the photons that build up in densityin the photonic crystal structure with the thin (preferably λ/4 opticalthickness or less) organic emitter material layer does not inducelasing, but does interact sufficiently to ensure that essentially alllight emission is stimulated in nature. Thus the emission isparticularly useful for display and lighting applications because thelight emitted from a BE-OLED is speckle-free emission. Speckle-freeemission is collimated light produced by stimulated emission that doesnot produce the well known speckle effect of visible laser light—that isthe ‘salt and pepper’ effect created by the destructive interference oflaser photons on a viewing surface.

The OLED contains an emitter material whose free spaceelectroluminescence emission yields a significantly high radiance at theband-edge wavelengths, that is to say, a radiance that when measurednormal to the device surface is preferably at least 25% and mostpreferably at least 50% of the radiance at the peak spectralelectroluminescence for the material. In other words, the measuredradiance of luminescence light emitted by the light emitting materialutilized in the organic light emitting diode is greater than one-quarterof the peak radiance of the luminescence emission spectrum of theemitter material measured normal to its light emitting surface. In otherwords, the emitter material in free space emits a substantial amount oflight in the wavelengths corresponding to the band-edge wavelengths ofthe photonic crystal. For the sake of simplicity this is referred to asthe emitter material emits light into the band-gap, or into theband-edge modes of the photonic crystal.

A particularly advantageous aspect of the disclosed devices is that thelocation of the emitter layer within the device need not be as preciseas is the case with the defect-mode devices.

In the following discussion, compare FIGS. 2 & 3 with FIGS. 5 & 6 . Therefractive index profile 510 of a simplified computer model of adisclosed BE-OLED device is shown in FIG. 5 . The device (not shown)described in this example consists of 43 layers or zones withalternating high (n₂) 520 and low (n₁) 530 refractive indices. (Thisnumber of layers is for exemplary purposes only and is not necessarilyan optimum number of device layers.) Zone 4 corresponds to a layercontaining light emitting material. Given this structure, when light isemitted from the emitter in zone 4 into band edge light propagationmodes that exist within the photonic crystal structure, the distributionof light intensity 610 within the device is that shown in FIG. 6 . Thisdistribution occurs because a very large number of modes exist and theynot only have multiple wavelengths, but also have different phaserelationships relative to the device layer boundaries for any particularemitted wavelength. This is in contrast to the FE-OLED devices describedby FIG. 2 and FIG. 3 wherein there is a single light propagation modeand a sharply peaked light intensity distribution at the center of thecentral layer or zone. When zones 1 through 7, including zone 4 of FIG.5 are projected onto the light intensity distribution 610 in FIG. 6 , itcan be seen that the emitter layer could be located anywhere in zone 4or, for that matter, in zones 2 through 6 with little impact on deviceperformance, because there is little difference in light intensity inany of these regions 2 through 6.

FIG. 7 illustrates cross-sections of various embodiments disclosed ofthe light emitting device 700. Light emitting device 700 comprises aphotonic crystal structure 702 which includes active zone 708 comprisinglayers 710, 712, 714 (indicated by the dotted lines), a non-limitingexample of which is a DBR, (not illustrated); a transmission axis 704;an emission surface 706; and an active zone 708. Active zone 708 isdisposed or formed within photonic crystal structure 702 and furthercomprises an organic layer 710, an anode 712, and a cathode 714. Theorganic layer further comprises a sublayer comprising an organicelectroluminescent material (not shown) and a sublayer comprising acharge transporting material (not shown). The organic layer 710 mayfurther comprise one or more additional sublayers (not shown) comprisingcharge carrier transport layers, charge carrier injection layers, chargecarrier blocking layers, thus it will be appreciated the organic layer710 may contain one or more additional OLED functional layers. It willbe appreciated that layers 710, 712 and 714 together comprise an OLED.As will be appreciated anode 712 and cathode 714 may each be a singleelectrode or alternatively may further comprise multiple layers ofelectrodes (not pictured). Active zone 708 is formed such that theperiodically varying index of refraction 750 along transmission axis 704of the photonic structure is not disrupted, or is substantially notdisrupted.

Preferably, the organic layer 710 has a maximum optical thickness ofapproximately ¼ of a single wavelength corresponding the centralwavelength prohibited within the photonic structure by the stop-bandcreated by the periodic index of refraction 750 (here the periodic indexof refraction is illustrated as a sinusoid, alternatively it may becloser to or substantially a square wave, or some other substantiallyperiodically varying index of refraction). If an optical thickness of ¼wavelength is impractical for a particular embodiment, the opticalthickness of the organic layer 710 may be equal to approximately % ofthe central wavelength of the stop-band. For example, the opticalthickness of each of layers 710, 712, and 714 is approximately ¼ of thecentral wavelength of the stop-band. When activated by a potentialapplied across anode 712 and cathode 714 the organic emitter moleculesare excited and photons are emitted into the band-edge modes of thephotonic crystal. As a result, visible light 716 is emitted from theemission surface 706 at wavelengths corresponding to the band-edgeemissions, for example in the bands 406 or 408 of FIG. 4 . Preferably,the molecules of the organic emitter material within the organic layer710 are spatially oriented to maximize stimulated emission parallel tothe transmission axis 704. As will be appreciated, when properly formed,the devices 700 being of a photonic structure having a stop-band, thedensity of states spectrum and transmissivity spectrum of device 700will be similar to those illustrated in FIG. 4 .

In various embodiments the active zone 708 comprises an organic layer710 that has a refractive index that is lower than that of the anodelayer 712 and cathode layer 714, and the anode layer and cathode layerrefractive indexes are higher than the adjacent portions of the photonicstructure. In various embodiments the anode and the cathode may beadjacent to opposite sides of the organic layer 710 and each may havethickness equivalent to ¼ wavelength of the central wavelength of thestopband, and the organic layer 710, comprising a sub-layer of lightemitting material, may have thickness equivalent to ¼ wavelength of thecentral wavelength of the stop-band and has a refractive index that islower than that of the anode layer and cathode layer.

FIG. 7 also illustrates various other embodiments of a disclosed lightemitting device 720. Similarly to device 700, device 720 comprises aphotonic crystal structure 722, which includes layers 732, 728, and 734(indicated by the dotted lines), a non-limiting example of which is, forinstance a DBR; a transmission axis 724; an emission surface 726; and anactive zone 728. Active zone 728 is disposed or formed within photonicstructure 722 and further comprises an organic layer 730. Photonicstructure 720 additionally includes two additional active zones 732 and734, each may have thickness equivalent to ¼ wavelength of the centralwavelength, respectively comprising an anode layer 732, and a cathodelayer 734. Organic layer 730 comprises an organic emitter material.Organic layer 730 may further comprise additional OLED functional layerssuch as charge carrier layers and charge injection layers (not shown).Organic layer 730 may also comprise one or more very thin metalliclayers, for example a first 0.5 nm cathode layer (not illustrated)formed from a 50:50 mixture of samarium and silver, or a very thincharge injection layer, as these layers may be formed of materialshaving relatively low index of refraction. As will be appreciated anode732 and cathode 734 may also each be a single electrode or alternativelymay further comprise multiple layers of electrodes (not pictured). Itwill be appreciated that one or more OLED functional layers may belocated within the anode layer 732 or cathode layer 734 depending ontheir respective index of refraction. Active zone 728 is formed suchthat the periodically varying index of refraction 750 along transmissionaxis 724 of the photonic structure is not disrupted. Preferably, theactive zone 728 has a thickness of approximately ¼ of a singlewavelength corresponding to the central mode prohibited within thephotonic structure due to the stop-band created by the periodic index ofrefraction 750. One physical difference between 700 and 720 is that theactive zone 728 extends planarly throughout two dimensions (i.e.horizontally across the cross section shown in FIG. 7 and into the pageof FIG. 7 ) of the photonic structure, whereas active zone 708 isentirely enclosed by the photonic structure (represented by the dottedlines of 708 not extending to the boundary of device 700). It will bethus appreciated that the active zones need not be layers, but moregenerally instead may be a zone confined to a small region of thehorizontal extent of the photonic crystal. Preferably, the zonecontaining the organic emitter material may be confined, or localized,within a region comprising less than 10% of the optical thickness of theentire photonic crystal in the vertical direction. It will beappreciated that the active zone may be smaller or larger than thepreferable thickness. It will be appreciated that the emitter materialmay be localized to a single layer comprising 10% or less of thevertical thickness of the photonic crystal 720.

In various disclosed embodiments the light emitting device 720 may be aphotonic crystal comprising eleven or more vacuum deposited layers.These eleven layers may first be four or more layers of dielectricmaterials having alternating indexes of refraction of a patternlow-high-low-high-low-high, then a fifth layer comprising an anode, asixth layer comprising an organic emitter material (or an OLED minuselectrodes), a seventh layer comprising a cathode and finally four ormore additional layers of dielectric materials having alternatingindexes of refraction of a pattern high-low-high-low-high-low, whereineach layer has substantially the same optical thickness. In variousdisclosed embodiments, the fifth, sixth and seventh layers comprising anOLED may further comprise various sublayers such as a hole injectionlayer, a hole transport layer, an emitter layer, an electron transportlayer, and an electron injection layer. In other embodiments the firstfour layers may be replaced by six or perhaps eight vacuum depositedlayers with alternating high and low indices of refraction, or the finalfour layers may be replaced by six or perhaps eight vacuum depositedlayers with alternating high and low indices of refraction. It will beappreciated that additional layers of alternating high and low indicesof refraction layers may be incorporated without deviating from theinvention. In various disclosed embodiments, the exemplary device, e.g.720, may be formed on a substrate 762, which may comprise a dielectricmaterial such as e.g. glass or plastic.

Various aspects of the devices disclosed herein may be formed usingsputtering techniques, or vacuum evaporation techniques, or othersimilar techniques as will be appreciated by one of skill in the art.Devices may also be formed using a combination of these techniques. Invarious embodiments, a band-edge type enhanced OLED, may have either ofthe structures shown in FIG. 7 . It is noted that the examplesillustrated in FIG. 7 are not drawn to scale. It is also noted that inthis example embodiment, thicknesses are approximate such that, e.g. 159nm, is preferably 159 nm, but may vary slightly given the limitations ofthe technologies used to form the constituent layers of the followingexample. Minor variations do not significantly impact performancebecause the device is forgiving—another improvement over cavity typedevices (e.g. FE-OLEDs) which are far more sensitive to variations.Non-limiting examples the structure of photonic crystal 720 may beformed according to the following exemplary embodiments.

In one preferred embodiment, photonic structure 722 may comprise aphotonic crystal. To form photonic structure 722, for example twosuccessive pairs of layers of dielectric material having alternatinghigh and low (relative to each other) index of refraction, each layerhaving 159 nm optical thickness, may be formed on transparent substrate762. Each pair may comprise a high index layer comprising TiO₂ and a lowindex layer which may comprise SiO₂. Non-limiting examples ofalternative low index layers may be formed of LiF, or MgF₂. Anon-limiting example of an alternative high index layer may be formed ofNb₂O₅. The high index layer in such a case would be formed adjacent to,or on, the substrate 762, which comprising a dielectric material such asglass or plastic will have a relatively low index of refraction. The lowindex layer would then be formed adjacent to the high index layer. Eachpair will be formed upon the previous pair such that the high indexlayer is formed adjacent to, or upon, the low index layer, thus forexample upon substrate 762, photonic crystal 722 may be formed firstsputtering one 71.7 nm layer of TiO₂ (optical thickness of 159 nm basedon a measured refractive index of 2.218) then sputtering one 108.8 nmlayer of SiO₂ (optical thickness of 159 nm based on a measuredrefractive index of 1.462), and repeating these steps two more times,such that this intermediate result comprises an uppermost layer having alow index of refraction.

Upon this intermediate result may be formed anode 732, for example anode732 may be a transparent inorganic semiconductor anode comprising a 79.2nm thick layer of In₂O₃ZnO (indium-zinc oxide, IZO, 90:10) (opticalthickness of 159 nm based on a measured refractive index of 2.008).Alternatively In₂O₃—SnO₂ (indium tin oxide, ITO) may be substituted forIZO. A ternary oxide such as gallium-indium-tin oxide may be used indevices meant to emit blue light since this material can have bettertransmissivity for blue light than the alternatives. Upon this anode 732may be formed an active layer 728 having an optical thickness of 159 nmcomprising, for example, the various organic materials and lowrefractive index material constituting the OLED (minus its electrodes),such that the index of refraction of active layer 728 is lower than theindex of refraction of the anode 732.

Continuing the example immediately above, active zone 728 may be formedfor example by thermal evaporation of the various constituentsub-layers, which may comprise a layer ofN,N′-Bis(naphthalen-1-yl)-N,N′-bis(phenyl)-benzidine (NPB) (forinstance, 31.8 nm in physical thickness equivalent to an opticalthickness of 65.6 nm based on a measured refractive index of 1.831)which material functions as a hole transporting material and a layer oftris-(8-hydroxyquinoline) aluminum (Alq3) (for instance, 47.8 nm inphysical thickness equivalent to an optical thickness of 92.3 nm basedon a measured refractive index of 1.724) which material functions asboth an electron transporting and an emissive material. Therefore theactive zone 728 of photonic structure 722 contains organic layer 730.The two constraints on active zone 728 in this exemplary embodiment arethat it have an optical thickness of approximately 159 nm, and the indexof refraction of the constituent layers are each respectively lower thanthe index of refraction of the anode layer 732. One or more additionalfunctional (for instance, metal and electron injection layers of thecathode) or non-functional layers may act as spacers if necessary toachieve the necessary thickness of active zone 728. Upon completion ofthis intermediate result, the photonic structure 722 comprises asubstrate having a relatively low index of refraction, three alternatingpairs of alternating dielectric layers of respectively high and lowindex of refraction, an anode layer having a high index of refractionand an active zone having a low index of refraction, where each layer ofthe intermediate result is approximately 159 nm in optical thickness.Optionally an electron injection layer of lithium fluoride (notillustrated) 0.5 nm in physical thickness (optical thickness of 0.6 nmbased on a refractive index of 1.294) may be formed by vacuum thermalevaporation may be formed upon the Alq₃ layer. This thin, relatively lowrefractive index layer have an optical thickness totaling 0.6 nm.

Continuing the example immediately above, a first 0.5 nm cathode layer(not illustrated) formed from a 50:50 mixture of samarium and silver byvacuum thermal evaporation may be deposited on top of the lithiumfluoride. This layer has an optical thickness of 0.5 nm and thereforehas a negligible effect on the refractive index profile of the photoniccrystal, and may be accounted for as part of the active layer 728 or thesecond cathode layer 734 A second cathode layer, for example 734, may beformed upon the first cathode layer. The second cathode layer 734 has anoptical thickness of approximately 159 nm thick may be fabricated fromsputtered IZO or another transparent conductive oxide. The secondcathode layer 734 has a relatively high index of refraction whencompared with the materials comprising the active zone 728, as well ashaving a high index of refraction when compared with SiO₂, or itsalternatives.

Upon the cathode layer two successive pairs of layers of dielectricmaterial having alternating low and high index of refractions may beformed, each layer approximately 150 nm thick. Each pair may comprise alow index layer which may comprise SiO₂, and a high index layercomprising TiO₂. As described above, non-limiting examples ofalternative low index layers may be formed of LiF, or MgF₂. Anon-limiting example of an alternative high index layer may be formed ofNb₂O₅. The low index layer in such a case would be formed adjacent to,or on top of, the cathode layer 734, which will have a relatively highindex of refraction. Each pair will be formed upon the previous pairsuch that the high index layer is formed adjacent to, or upon, the lowindex layer, thus for example upon cathode layer 734, photonic crystal722 may continue to be formed by sputtering one 159 nm layer of SiO₂then sputtering one 159 nm layer of Nb₂O₅, and repeating this one moretime, such that this final result comprises a single photonic crystal722 having an uppermost layer having a high index of refraction. It willbe appreciated that the emitter material within the photonic crystal isconfined to a region that is less than 10% of the overall opticalthickness of the device.

In another alternative exemplary embodiment, exemplary photonic crystal,e.g. 720, may instead be formed as follows. Beginning with transparentsubstrate 762 comprising glass or plastic. As with the embodimentdescribed above, three successive pairs of layers of dielectric materialhaving alternating high and low (relative to each other) index ofrefraction, each layer 147.5 nm in optical thickness may be formed onthe substrate. Each pair may comprise a high index layer comprisingNb₂O₅ and a low index layer which may comprise SiO₂. Non-limitingexamples of alternative low index layers may be formed of LiF, or MgF₂.A non-limiting example of alternative high index layers may be formed ofTiO₂. The high index layer in such a case may be formed adjacent to thesubstrate, which comprising glass or plastic will have a relatively lowindex of refraction. Each pair will be formed upon the previous pairsuch that the high index layer is formed adjacent to, or upon, the lowindex layer, thus for example upon substrate 762, photonic crystal 722may be formed first sputtering one 82.2 nm layer of Nb₂O₅ (opticalthickness of 147.5 nm based on a measured refractive index of 1.795)then sputtering one 101.2 nm layer of SiO₂ (optical thickness of 147.5nm based on a measured refractive index of 1.457), and repeating thistwo more times, such that this intermediate result comprises anuppermost layer having a low index of refraction.

As in the previous embodiment described above, upon this intermediateresult may be formed anode 732, for example anode 732 may be atransparent inorganic semiconductor anode comprising a 73.5 nm thicklayer of In₂O₃—ZnO (indium-zinc oxide, IZO) (optical thickness of 147.5nm based on a measured refractive index of 2.008). Alternatively theanode 732 may comprise a layer of optical thickness 147.5 nm comprisingapproximately 10% ZnO₂ and 90% In₂O₃. Upon this anode 732 may be formedan active layer 728 having a total optical thickness of 147.5 nmcomprising, for example, the various organic and other low refractiveindex materials constituting the OLED, such that the index of refractionof active layer 728 is lower than the index of refraction of the anode732.

Continuing the example immediately above, active zone 728 may be formedfor example by thermal evaporation of the various constituent layers,which in this case may comprise (preferably in the following order): athermally evaporated layer of copper phthalocyanine (CuPC) 2.0 nm inthickness (2.6 nm in optical thickness based on a refractive index of1.318), or alternativelydipyrazino[2,3-f:2′,3′-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile(HAT-CN), which functions as a hole injection layer; followed by a 35.0nm thick thermally evaporated layer of4,4′-Bis(9-carbazolyl)-1,1′-biphenyl (CBP) (64.4 nm in optical thicknessbased on a refractive index of 1.840) which material functions as a holetransporting material; a 15.0 nm thick layer of thermally evaporated CBPdoped with 8% bis[2-(2-pyridinyl-N)phenyl-C](acetylacetonato)iridium(III) (Ir(ppy)₂(acac)) (27.5 nm in opticalthickness based on a refractive index of 1.831) which material functionsas the emissive material; a 29.3 nm thick layer of2,2′,2″-(1,3,5-benzinetriyl-tris(1-phenyl-1-H-benzimidazol (TPBi) (50.8nm in optical thickness based on a refractive index of 1.736) whichfunctions as an electron transport layer; and a 1.0 nm layer of8-hydroxyquinolatolithium (Liq) (1.7 nm in optical thickness based on arefractive index of 1.7), which functions as an electron injectionlayer. Therefore the active zone 728 contains organic layer 730. The twoconstraints on active zone 728 in this exemplary embodiment are that ithave a thickness of approximately 147.5 nm, and the index of refractionof the constituent layers are each respectively lower than the index ofrefraction of the anode layer 732. One or more additional functional(for instance, metal and electron injection layers of the cathode) ornon-functional layers may act as spacers if necessary to achieve thenecessary thickness of active zone 728. Upon completion of thisintermediate result, the photonic crystal 722 comprises a substratehaving a relatively low index of refraction, three alternating pairs ofalternating dielectric layers of respectively high and low index ofrefraction, an anode layer having a high index of refraction and anactive zone having a low index of refraction, where each layer of theintermediate result is approximately 147.5 nm in thickness.Additionally, the hole injection layer and hole transport layers,respectively comprising CuPC and CBP in this example may be substitutedwith one layer ofN,N′-Bis-(1naphthalenyl)-N,N′-bis-phenyl-(1,1′-biphenyl)-4,4′-diamine(NBP), which is a hole transport material having electron blockingproperties.

Continuing the example immediately above, a first 0.5 nm cathode layer(not illustrated) formed from a 50:50 mixture of samarium and silver byvacuum thermal evaporation may be deposited on top of the8-hydroxyquinolatolithium. This layer has an optical thickness of 0.5nm. A second cathode layer, for example 734, may be formed upon thefirst cathode layer. The second cathode layer 732 has an opticalthickness of approximately 147.5 nm thick may be fabricated fromsputtered IZO or another transparent conductive oxide. The secondcathode layer 734 has a relatively high index of refraction whencompared with the materials comprising the active zone 728, as well ashaving a high index of refraction when compared with SiO₂, or itsalternatives.

Upon the cathode layer three successive pairs of layers of dielectricmaterial having alternating low and high index of refractions may bedeposited, each with an optical thickness of approximately 147.5 nm.Each pair may comprise a low index layer which may comprise SiO₂, and ahigh index layer comprising Nb₂O₅. As described above, non-limitingexamples of alternative low index layers may be formed of quartz, LiF,or MgF₂. A non-limiting example of an alternative high index layer maybe formed of TiO₂. The low index layer in such a case may be formedadjacent to, or on top of, the cathode layer 734, which will have arelatively high index of refraction. Each pair will be formed upon theprevious pair such that the high index layer is formed adjacent to, orupon, the low index layer, thus for example upon cathode layer 734,photonic crystal 722 may continue by sputtering one approximately 147.5nm layer of SiO₂ then sputtering one approximately 147.5 nm layer ofNb₂O₅, and repeating this two more times, such that this final resultcomprises a single photonic crystal 722 an uppermost layer having a highindex of refraction.

In another alternative exemplary embodiment, exemplary photonic crystal,e.g. 720, may instead be formed as follows. Beginning with transparentsubstrate 762 comprising glass or plastic. Two successive pairs oflayers of dielectric material having alternating high and low (relativeto each other) index of refraction, each layer 162 nm in opticalthickness may be formed on the substrate. Each pair may comprise a highindex layer comprising TiO₂ and a low index layer which may compriseSiO₂. Non-limiting examples of alternative low index layers may beformed of LiF, or MgF₂. A non-limiting example of alternative high indexlayers may be formed of Nb₂O₅. The high index layer in such a case maybe formed adjacent to the substrate, which comprising glass or plasticwill have a relatively low index of refraction. Each pair will be formedupon the previous pair such that the high index layer is formed adjacentto, or upon, the low index layer, thus for example upon substrate 762,photonic crystal 722 may be formed first sputtering one 73.4 nm layer ofTiO₂ (optical thickness of 162 nm based on a measured refractive indexof 2.206) then sputtering one 110.7 nm layer of SiO₂ (optical thicknessof 162 nm based on a measured refractive index of 1.463), and repeatingthis one more time, such that this intermediate result comprises anuppermost layer having a low index of refraction.

As in the previous embodiment described above, upon this intermediateresult may be formed anode 732, for example anode 732 may be atransparent inorganic semiconductor anode comprising a 82.4 nm thicklayer of In₂O₃—ZnO (indium-zinc oxide, IZO) (optical thickness of 162 nmbased on a measured refractive index of 1.966). Alternatively the anode732 may comprise a layer of optical thickness 162 nm comprisingapproximately 10% ZnO₂ and 90% In₂O₃. Upon this anode 732 may be formedan active layer 728 having a total optical thickness of 162 nmcomprising, for example, the various organic and other low refractiveindex materials constituting the OLED, such that the index of refractionof active layer 728 is lower than the index of refraction of the anode732.

Continuing the example immediately above, active zone 728 may be formedfor example by thermal evaporation of the various constituent layers,which in this case may comprise: a thermally evaporated layer ofN,N′-Bis(32aphthalene-1-yl)-N,N′-bis(phenyl)-benzidine (NPB) (forinstance, 20 nm in physical thickness equivalent to an optical thicknessof 35 nm based on a measured refractive index of 1.748) which functionsas a hole injection and transporting layer; followed by a 5.0 nm thickthermally evaporated layer of 4,4′,4″-tri(N-carbazoyl)triphenylamine(TcTa) (9.0 nm in optical thickness based on a refractive index of1.807) which material functions as a triplet blocking material; a 15.0nm thick layer of thermally evaporated9-(3-(3,5-di(pyridine-2-yl)-1H-1,2,4-triazol-1-yl)phenyl-9H-carbazole(m-CBTZ) doped with 10%bis(2-phenylbenzothiazolato)(acetylacetonato)iridium (III)(Ir(bt)₂(acac) (29.0 nm in optical thickness based on a refractive indexof 1.935) which material functions as the emissive material; a 51.4 nmthick layer of TPBi (87.8 nm in optical thickness based on a refractiveindex of 1.708) which functions as an electron transport layer; and a0.5 nm layer of lithium fluoride (0.7 nm in optical thickness based on arefractive index of 1.391), which functions as an electron injectionlayer. Therefore the active zone 728 contains organic layer 730. The twoconstraints on active zone 728 in this exemplary embodiment are that ithave a thickness of approximately 162 nm, and the index of refraction ofthe constituent layers are each respectively lower than the index ofrefraction of the anode layer 732. One or more additional functional(for instance, metal and electron injection layers of the cathode) ornon-functional layers may act as spacers if necessary to achieve thenecessary thickness of active zone 728. Upon completion of thisintermediate result, the photonic crystal 722 comprises a substratehaving a relatively low index of refraction, two alternating pairs ofalternating dielectric layers of respectively high and low index ofrefraction, an anode layer having a high index of refraction and anactive zone having a low index of refraction, where each layer of theintermediate result is approximately 162 nm in thickness.

Continuing the example immediately above, a first 0.5 nm cathode layer(not illustrated) formed from a 50:50 mixture of samarium and silver byvacuum thermal evaporation may be deposited on top of the lithiumfluoride. This layer has an optical thickness of 0.5 nm. A secondcathode layer, for example 734, may be formed upon the first cathodelayer. The second cathode layer 732 has an optical thickness ofapproximately 162 nm thick may be fabricated from sputtered IZO oranother transparent conductive oxide. The second cathode layer 734 has arelatively high index of refraction when compared with the materialscomprising the active zone 728, as well as having a high index ofrefraction when compared with SiO₂, or its alternatives.

Upon the cathode layer three successive pairs of layers of dielectricmaterial having alternating low and high index of refractions may bedeposited, each with an optical thickness of approximately 162 nm. Eachpair may comprise a low index layer which may comprise SiO₂, and a highindex layer comprising TiO₂. As described above, non-limiting examplesof alternative low index layers may be formed of LiF, or MgF₂. Anon-limiting example of an alternative high index layer may be formed ofNb₂O₅. The low index layer in such a case may be formed adjacent to, oron top of, the cathode layer 734, which will have a relatively highindex of refraction. Each pair will be formed upon the previous pairsuch that the high index layer is formed adjacent to, or upon, the lowindex layer, thus for example upon cathode layer 734, photonic crystal722 may continue by sputtering one approximately 162 nm layer of SiO₂then sputtering one approximately 162 nm layer of Nb₂O₅, and repeatingthis two more times, such that this final result comprises a singlephotonic crystal 722 an uppermost layer having a high index ofrefraction.

In the exemplary embodiments disclosed above, one of skill in the artwill appreciate that light will be emitted from each “end”, or surface726, of the photonic crystal, 722, and approximately parallel to thetransmission axis 736. To configure the device to emit from only oneend, a mirror or reflector needs to be placed on one end. When metallicmirrors are used allowance should be made for the phase shift thatoccurs upon reflection from a metallic surface, for example, byincreasing the thickness of the adjacent layer. Alternatively it will bewithin the skill of one skilled in the art, to simply increasing thenumber of high-index/low-index dielectric pairs at one end of the deviceuntil no light or very little light is emitted from that respective end.

One of skill in the art will appreciate that small changes in the phaseof light caused by the metallic cathode, various layer boundaries, andunknown or uncontrollable variations in the indices of refraction mayrequire tuning of the thicknesses in various components. Thesevariations will occur on a determinate basis, and therefore given thechosen materials one can tune the thicknesses of one or more layers,such as a layer of TBP, to correct for these minor variations.Additionally, when sputtering ITO and IZO, variations in the index ofrefraction will occur in these materials, which can also be accountedfor by tuning the physical thickness in order to achieve the correctoptical thickness.

The light that is emitted from the photonic crystal structures of theinventive devices propagates in the direction normal to plane of thedevice (parallel to the transmission axis 704). The electric vectorsassociated with this light are therefore all oriented parallel to theplane of the device. This means that this light will only stimulateemission from those excited state molecules having transition momentswith components substantially in the plane of the device. Energy that isused to excite molecules whose transition moments are substantiallyperpendicular to the plane of the device may therefore is lost toin-plane light emission or non-radiative relaxation mechanisms. For thisreason, host-dopant mixtures in which an anisotropic host preferentiallyaligns the transition moments of the emissive dopants in the plane ofthe device are preferred. An example is CBP doped with Ir(ppy)₂(acac).

One of skill in the art will also appreciate that various other aspectsmay have different layer compositions. Various additional embodiments ofa disclosed device 800 are portrayed in FIG. 8 . The shown embodimentsmay comprise a photonic crystal having a band-gap, and be formed ofalternating high index of refraction dielectric materials and low indexof refraction dielectric materials capable of producing a periodicallyvarying refractive index, and an OLED containing an emitter materialwhose free space electroluminescence emission yields a significantlyhigh radiance at the band-edge wavelengths, that is to say, a radiancethat when measured normal to the device surface is preferably at least25% and most preferably at least 50% of the radiance at the peakspectral electroluminescence for the material. In other words, themeasured radiance of luminescence light emitted by the light emittingmaterial utilized in the organic light emitting diode is greater thanone-quarter of the peak radiance of the luminescence emission spectrumof the emitter material measured normal to its light emitting surface.The device comprises three sub-structures: a first portion of thephotonic crystal structure 810, a central low refractive index zone 812,and a second portion of the photonic crystal structure 814. Thereferences to “a portion” are meant as convention to ease thedescription the components of 800 which is formed to be a single unitaryphotonic crystal. Aside from these three sub-structures the devicesdescribed by FIG. 8 may also comprise a transparent anode 822, a thinfirst cathode layer 834 composed of a low work function metal, and asecond transparent cathode layer 836. The transparent anode 822 may befabricated from any suitable transparent conductive material such asindium-tin oxide or indium-zinc oxide. The first cathode layer may befabricated from thin, transparent film of any suitable low work functionmetal, for instance aluminum, a magnesium/silver alloy, silver/rareearth alloy or a pure rare earth metal such as samarium or ytterbium.The second cathode layer 836 may be fabricated from any suitabletransparent conductive material such as indium-tin oxide. A second metallayer may also optionally be inserted between layers 834 and 836, forinstance, if the materials in 834 and 836 are not compatible with eachother.

The first portion of the photonic crystal structure 810 may comprisemultiple (in this non-limiting example five, but can be more or less)layer pairs 816. Each of the layer pairs is comprised of a layer 818 ofa transparent high refractive index material and a layer 820 of atransparent low refractive index material. Each of the layers thatcomprise the layer pairs 816 have an optical thickness equal toone-quarter of the central wavelength of the stop-band of the photoniccrystal sub-structure 810. Optical thickness being equal to the physicalthickness of the layer times the refractive index of the layer. Thesecond portion of the photonic crystal structure 814 may comprisemultiple (in this non-limiting example five, but can be more or less)layer pairs 838. Each of the layer pairs may comprise a layer 840 of atransparent low refractive index material and a layer 842 of atransparent high refractive index material. Each of the layers thatcomprise the layer pairs 838 may have an optical thickness equal toabout one-quarter of the central wavelength of the stop-band of thephotonic crystal sub-structure 814 which is in turn equal to that ofphotonic crystal sub-structure 810.

The central low refractive index zone 812 may comprise a hole injectionlayer 824, a hole transporting layer 826, an emitter layer 828, anelectron transporting layer 830, and an electron injection layer 832.All of the layers contained in the central low refractive index zone 812have refractive indices lower than those of electrodes 822 and 836 andthe total optical thickness of all the layers contained in zone 812 isequal to one-quarter of the central wavelength of the stopbands of thephotonic crystal sub-structure 810 and 814. In various embodiments theyemitter layer 828 comprises an emitter material having a emissionspectrum and an absorption spectrum, and the band-gap is tailored (byaltering the optical thickness of each layer of the device, or moregenerally by altering the physical length of the spatial period of theperiodic refractive index profile) such that the peak radiancewavelength of the band-edge light emission at the band-edge of theband-gap and measured normal to the device surface is a wavelength atwhich free space light emission of the emitter material is preferablygreater than ¼ and most preferably greater than ½ the peak radiance ofthe emitter. The emitter material whose free space electroluminescenceemission yields a significantly high radiance at the band-edgewavelengths, that is to say, a radiance that is preferably at least 25%and most preferably at least 50% of the radiance at the peak spectralelectroluminescence for the material.

Electrode layers 822 and 836 may have refractive indices that are notonly higher than the materials in the central low refractive index zone812, but they may also have refractive indices that are higher thanadjacent layers 820 and 840 respectively. Electrode layers 822 and 836may also have optical thickness equal to one-quarter of the centralwavelength of the stop-band of the photonic crystal sub-structure 814and 816. Cathode layer 834 may be extremely thin and generally has anegligible effect on the optical thickness of cathode 836, but should becounted as part of either central low index zone 812 or cathode layer836 (which ever of the two its index is closer to) in terms of opticaldesign. In this way the sequence of layers 820, 822, 812 (compositelayer), 836, and 840 may yield the low/high/low/high/low alternation ofrefractive indices required for inclusion in a photonic crystal. Thus itcan be seen that sub-structures 810, 812, 814 and layers 822 and 836 allmay as a unitary combination form a single photonic crystal structure800.

When device 800 is electrically activated, holes flow from anode 822through hole injection layer 824 and hole transporting layer 826 intoemitter layer 828. At the same time electrons flow from cathode layers834 and 836 through electron injection layer 832 and electrontransporting layer 830 into emitter layer 828. The electrons and holesrecombine on luminescent material molecules in layer 828 yieldingexcitons. Since emitter layer 828 may be inside a photonic crystalstructure, excitons created in that layer cannot emit light atwavelengths in the stop-band of the photonic crystal. However, where theemission band of the luminescent material in layer 828 overlaps theband-edge wavelengths of the stop-band, light emission does occur andbecause of the high density of states at those wavelengths unusuallyhigh levels of emission occur. The photonic crystal traps the light fromband-edge emission within its structure increasing the photon density tothe point where there are sufficient photons interacting with excitonsthat nearly all light emission is stimulated emission. There is,however, by design and usually because of the nature of the materialsinvolved insufficient laser gain in the organic materials to supportlasing at current levels achievable in these devices. Since the lightfrom stimulated emission is almost completely vertical in its directionof propagation within the device, there is very little loss due tointernal reflection and trapping of light and the device is as a resulthighly energy efficient.

The nature and number of the layers comprising central low refractiveindex zone 812 may be altered so long as there is a emitter layerpresent that may be electrically activated to emit light and as long asthe emission spectrum of that emitter material contain wavelengths thatoverlap the stop-band of the photonic crystal. For instance, thefunctions of hole injection layer 824 and hole transporting layer may becombined into a separate single layer. The functions of electroninjection layer 832 and electron transporting layer 830 may be combinedinto a separate single layer. Additional hole transporting or electrontransporting as well as hole blocking, electron blocking, and tripletblocking layers may be introduced.

The photonic crystal structures in device 800 may be built up a layer ata time as are the functional OLED layers of the device. Thus thephotonic crystal structures may have a discontinuous periodic refractiveindex profile.

In some cases the proper electrical functioning of the device 800 mayrequire that the total thickness of central low refractive index zone812 be greater than one-quarter the desired central wavelength of thestop-band of the photonic crystal 800. For instance, this issue may, butnot necessarily, occur in devices that are designed to produce blue orviolet light, or any other color. If this is the case, the thickness ofcentral low refractive index zone 812 may total three quarters of thecentral wavelength of the stop-bands of the photonic crystalsub-structures 810 and 814 in optical thickness. One will appreciatethat the number of alternating layers can be more or less than thosedescribed in FIG. 8 , and that the optimum number of layers may differdepending on the application.

Increasing the thickness of zone 812 to three-quarters of the centralstop-band wavelength may not be the best solution for blue or violetemitting devices. Instead an alternative solution is the altered design900 as shown in FIG. 9 . Device 900 is quite similar to device 800 inthat there is a first portion of the photonic crystal structure 910, acentral low refractive index zone 912, and a second portion of thephotonic crystal structure 914 with these three sub-structurescorresponding to sub-structures 810, 812, and 814 in device 800. Thedifference here is that central low refractive index zone 912 comprisesonly emitter layer 928, electron transporting layer 930, and electroninjection layer 932. Hole injection layer 924 combines with holetransporting layer 926 to compose another low refractive index zone 944.This low refractive index zone is separated from the central lowrefractive index zone 912 by a second hole transporting layer 946. Thematerial in the hole transporting layer 946 has a higher refractiveindex than the materials in zones 944 and 912. The combined opticalthickness of the layers that compose zone 912 is equal to one-quarter ofthe central wavelength of the stop-bands of the photonic crystalsub-structures 910 and 914. The combined optical thickness of the twolayers that compose zone 944 is equal to one-quarter of the centralwavelength of the stop-bands of the photonic crystal sub-structures 910and 914, and the optical thickness of layer 946, of anode 922, and ofcathode layer 936 are each equal to one-quarter of the centralwavelength of the stopbands of the photonic crystal sub-structures 910and 914. Thus it can be seen that first portion of the photonic crystalstructure 910, the low refractive index zone 944, the central lowrefractive index zone 912, and the second portion of the photoniccrystal structure 914 combine with layers 922, 946, and 936 to create asingle photonic crystal structure. This structure interacts with lightemitted by emitter layer 928 in the same manner as described for device800 above.

Additional alternative embodiments are illustrated by device 1000 thatalso may solve the potential issues inherent with generating shortwavelength light is shown in FIG. 10 . This device has a first portionof the photonic crystal structure 1010, a central low refractive indexzone 1012, and a second portion of the photonic crystal structure 1014with these three structures corresponding to structures 810, 812, and814 in device 800. The difference in this device is that central lowrefractive index zone 1012 comprises only emitter layer 1028, holetransporting layer 930, and hole injection layer 1032. Electroninjection layer 1024 combines with electron transporting layer 1026 tocompose another low refractive index zone 1044. This low refractiveindex zone is separated from the central low refractive index zone 1012by a second electron transporting layer 1046. The material in electrontransporting layer 1046 has a higher refractive index than the materialsin zones 1044 and 1012. The combined optical thickness of the layersthat compose zone 1012 is equal to one-quarter of the central wavelengthof the stop-bands of the photonic crystal sub-structures 1010 and 1014.The combined optical thickness of the two layers that compose zone 1044is equal to one-quarter of the central wavelength of the stop-bands ofthe photonic crystal sub-structures 1010 and 1014, and the opticalthickness of layer 1046, of anode 1022, and of cathode layer 1036 areeach equal to one-quarter of the central wavelength of the stop-bands ofthe photonic crystal sub-structures 1010 and 1014. Thus it can be seenthat the first portion of the photonic crystal structure 1010, lowrefractive index zone 1044, central low refractive index zone 1012, andthe second portion of the photonic crystal structure 1014 combine withlayers 1022, 1046, and 1036 to create a single photonic crystalstructure. This structure interacts with light emitted by emitter layer1028 in the same manner as described for device 800 above.

Devices such as 800, 900, and 1000 are unlike known devices because theyare incapable of producing laser light, have no micro-cavity in whichlight generation occurs, utilize stop-band edge stimulated emission, andhave photonic crystals with discontinuous refractive index profiles. Atthe present time this may be the only combination of device propertiesthat enables the production of commercially feasible OLED devices withvery high energy efficiency.

FIG. 11 illustrates how to choose a combination of emitter material andband-gap material. Illustrated is an exemplary transmission spectrum1110 of a photonic crystal having a band-gap 1120. Also illustrated arethe absorption spectrum 1130 and the emission spectrum 1140 of anexemplary emitter material. As is shown the emission spectrum has a peakpower, 1150 occurring at λ_(PP), a half peak power 1160 occurring atλ_(1/2PP), and a one-quarter peak power 1195 occurring at κ_(1/4PP).Preferably, to achieve a high efficiency device, the photonic crystal isconfigured such that an edge of the band-gap 1170 falls between the peakemission wavelength 1150 and the ¼ peak emission wavelength 1195, in aregion of the spectrum that overlaps areas of the absorption spectrum1130 as little as possible, in other words, in a region 1180 where thereis low absorption by the emitter material. Most preferably the photoniccrystal is configured such that an edge of the band-gap 1170 fallsbetween the peak emission wavelength 1150 and the ½ peak emissionwavelength 1160, in a region of the spectrum that overlaps areas of theabsorption spectrum 1130 as little as possible, in other words, in aregion 1180 where there is low absorption by the emitter material. Theband-gap edge 1170 may fall on a wavelength at which light absorptionfor a single pass of light through the emitter layer is less than 1%.Preferably, the band edge 1170 may fall on a wavelength at which lightabsorption for a single pass of light through the emitter layer is lessthan ½%, while also corresponding to a wavelength of the emissionspectrum that is greater than ¼ peak radiance.

1190. Also, it will be appreciated that the emission spectrum of anemitter material may have more than one peak, and that hereindiscussions of peak radiance are relative to the region in the spectrumnearby to the band-gap. It will be appreciated that the output light ofthe disclosed device is determined based on the wavelengthscorresponding the band-edge, such that the output spectrum of the devicecan be tailored by adjusting the layer thicknesses and thus theband-edge wavelengths. Because a first photonic crystal having astop-band corresponding to one wavelength may be transparent, orsubstantially transparent to a second photonic crystal having astop-band that is shifted up or down in the transmission spectrum fromthe first photonic crystal, a single device capable of emittingband-edge light corresponding to two or more stop-bands may be formed bystacking one or more photonic crystals atop each other.

Often, emitter material molecules in OLED devices have shapes that favorlight emission in some directions relative to molecular orientation overothers. Because of this, by uniformly aligning the molecules of theemitter material in an emitter layer in a specific orientation, lightwill be emitted more intensely in some directions relative to the planeof the emitter layer than in others. This is because the molecularorbitals in these molecules may be asymmetric in shape and in terms oftheir electronic polarizability. Interaction with incident light oremission of light will be strongest when the electric vector of thelight lies along the direction of highest electrical polarizabilitywithin an orbital. However, since light emission or absorption involvestwo molecular orbitals, that of the excited and that of the electronicground state, and since quite often the directions of highest electricalpolarizability are different for these two orbitals, the light electricvector direction yielding the highest interaction or emission isintermediate between the directions of highest electron polarizabilityfor the two orbitals. The light electric vector direction that yieldsthe highest interaction is termed the transition moment (or transitiondipole moment) since it is the direction of the transient electricdipole induced in the molecule by interaction with the light (orconversely by the direction the transient electric dipole that emits thelight). Thus it can be seen that if the emitter molecules are uniformlyaligned such that their transition moments are perpendicular to thepropagation direction of the feedback light, the efficiency of theinteraction between emitter materials and the feedback light ismaximized thereby producing maximum stimulated emission. This sort ofalignment can be achieved by utilizing rigid anisotropic emittermolecules of the right geometry dissolved in host materials that haverigid rod or disk-shaped molecules that “lay down” on the underlyinglayer surface thus yielding and anisotropic environment that, in turn,aligns the emitter molecules. Host materials that may exhibit thisbehavior are 4,4′-bis(carbazol-9-yl)biphenyl (CBP) andN,N′-bis(naphthalene-1-yl)N,N′-bis(phenyl)benzidine (NPB).

Some iridium III organometallic phosphorescent emitter materials mayhave their molecules spontaneously aligned by some host materials so asto have their transition moments for the desired phosphorescent emissionpredominantly aligned in the plane of the OLED emissive layer. Inparticular some heteroleptic iridium III complexes with two ligandscomprising aromatic substituted nitrogen containing aromatic compoundsand third acetoacetonate ligand have displayed this sort of alignment.Examples of this are bis(2phenylpyridine) (acetylacetonate)iridium(III)dopant in a 4,4′-bis(carbazol-9-yl)biphenyl host andbis(2methyldibenzo[f,h]quinoxaline) (acetylacetonate)iridium(III) inN,N′-bis(naphthalene-1-yl)N,N′-bis(phenyl)benzidine. Anisotropic emittermaterial formulations such as these can be used advantageously tofurther increase the energy efficiency of the devices of this invention.

The embodiments described above are illustrative examples and it shouldnot be construed that the present invention is limited to theseparticular embodiments. For example, although OLED devices were used asexamples of emissive devices, other luminescent material or structuresmay be used, not limited to OLEDs. Further, although refractive indexprofiles, direction of light, etc. were described as being “normal” to aplane, it should be understood that they need not be exactly normal,rather in a close range of being normal or substantially normal.Accordingly, the embodiments described in this application also mayinclude cases in which they are about normal or substantially normal toa plane. Further, various components and aspects described withreference to different embodiments are interchangeable among differentembodiments, and are not limited to one particular embodiment. Thus,various changes and modifications may be effected by one skilled in theart without departing from the spirit or scope of the invention asdefined in the appended claims.

While the present invention has been particularly shown and describedwith reference to example embodiments thereof, it will be understood bythose of ordinary skill in the art that various changes in form anddetails may be made therein without departing from the spirit and scopeof the present invention as defined by the following claims andequivalents thereof. It is therefore desired that the presentembodiments be considered in all respects as illustrative and notrestrictive, reference being made to the appended claims and equivalentsthereof rather than the foregoing description to indicate the scope ofthe invention.

We claim:
 1. A single light emitting photonic crystal having an organicelectroluminescent emitter material disposed within the single photoniccrystal, wherein the organic electroluminescent emitter materialcomprises an organic light emitting material localized in a zone havingless than 10% of an optical thickness of the photonic crystal, whereinthe organic electroluminescent emitter material has a free spaceemission spectrum that at least in part overlaps a stop-band of thephotonic crystal, wherein the photonic crystal emits light at awavelength corresponding to an edge of the stop-band that the organicelectroluminescent emitter material overlaps, wherein the photoniccrystal further comprises a stack of layers of varying refractive index,wherein a layer of lower index of refraction materials comprises theorganic electroluminescent emitter material.
 2. The single lightemitting photonic crystal of claim 1 wherein the stack of layers has aperiodic modulation, measured in optical thickness, of refractive indexwith a period of modulation of λ/2 wherein λ equals the centralwavelength of the photonic crystal stop-band.
 3. The single lightemitting photonic crystal of claim 1 wherein the stack of layers ofvarying refractive index comprised by the single light emitting photoniccrystal comprises at least one pair of layers of materials withdifferent refractive indices that, in turn, comprises a first layer ofmaterial with a higher refractive index and a second layer of a materialwith a lower refractive index, wherein the total optical thicknesses ofthe two layers combined equals λ/2, wherein λ equals the centralwavelength of the photonic crystal stop-band.
 4. The single lightemitting photonic crystal of claim 2 wherein the period of therefractive index modulation through the stack of layers is disrupted byinsertion of a layer of constant refractive index having an opticalthickness equal to 3λ/4 wherein λ is the central wavelength of thestop-band of the photonic crystal.
 5. The single photonic crystal ofclaim 1, wherein the edge of the stop-band occurs at a wavelength atwhich measured radiance of free space luminescent light emission by theorganic electroluminescent emitter material is greater than one-quarterof the peak radiance of the free space luminescent emission spectrum ofthe emitter material.